Mid-Miocene sedimentation across the highly extended Colorado River extensional corridor (CREC) demonstrates how the 3-D geometry and evolution of the regional detachment fault system and overlying listric normal and oblique-slip faults controlled the age, distribution, and fill of coeval supradetachment basins. Basin development followed a temporal evolution in the regional slip direction (NE), with minimal, oldest, siliciclastic sedimentation in the breakaway region (SW) and younger deposits farther northeast, all in the hanging wall of the detachment fault system. The thickest (≤2.5 km) and longest-lived deposition was focused in the center of the CREC, adjacent to the Chemehuevi core complex. Proximal alluvial fan deposition dominated, with lesser distal fan, braidplain, and lacustrine sedimentation. Megabreccia, or rock avalanche, deposits are locally abundant. Syntectonic basins were small (less than a few kilometers across) and isolated from each other, hosting locally derived debris and rapid vertical and lateral facies changes. Basin location, shape, and depositional history were determined by the geometry of the corrugated detachment fault below, permitting thick accumulations in troughs parallel to regional extension (SW-NE). Displacement on listric normal faults in the detachment system hanging wall created elongate half-graben basins perpendicular to regional extension (NW-SE). Drag on oblique-slip faults bordering tilted hanging wall blocks created accommodation space and folded and displaced strata during and after deposition. Depocenters moved as faulting continued. New 40Ar/39Ar ages on interbedded volcanic deposits show sedimentation occurred between ca. 16 Ma and 14 Ma. 3-D analysis of these early synrift supradetachment basins contributes to understanding strata buried deep below nonvolcanic passive continental margins.

Oceanic core complexes (OCCs) are a fundamental component of slow-to-ultraslow spreading mid-ocean ridges, yet the processes that control OCC formation and evolution are poorly understood especially with respect to their high-temperature lithospheric roots. We present detailed analyses of high-temperature ductile deformation preserved in drill-core from IODP Hole U1601C, on the Atlantis Massif OCC (30°N, MAR). We show that gabbroic intrusions within peridotite accommodated significant high-temperature deformation, especially within Fe-Ti oxide-bearing assemblages. This deformation spatially localizes in zones of high lithological heterogeneity created by meter-to-submeter-scale gabbroic intrusions within peridotite. High-temperature ductile deformation often localizes close to, and/or along, intrusive contacts, accompanied by localized, evolved, melt-reactive porous flow (crystallizing Fe-Ti oxides), and followed by fluid-rock reaction that enhanced and sustained further ductile deformation. These spatially controlling relationships between magmatism, deformation, and late melt ± fluid infiltration are a direct consequence of the lithological heterogeneity within moderately-magmatic OCCs, which are the dominant style of OCC along the Mid-Atlantic Ridge and other slow-spreading ridges.

Supplemental Text S1: Details of tuff correlation, central Chemehuevi Valley. Supplemental Text S2: Explanation of domain rotations and translations in Figure 18. Figures S1–S4: Figures illustrating selected features and maps. Table S1: Additional data to augment Table 3. Table S2: Details of domain rotations and translations.

Water that flows through permeable ultramafic rocks produces high abundances of molecular hydrogen (H-2), methane (CH4), and other small organic molecules. Such products can fuel life in the rocky subseafloor, be extracted for energy, and may have played a role in pre-biological chemical synthesis on early Earth or other planetary bodies. The International Ocean Discovery Program drilled a new 1268-m-deep borehole (U1601C) into serpentinized mantle with minor gabbroic rocks on the Atlantis Massif, similar to 800-m north of the Lost City hydrothermal field (30 degrees N, Mid-Atlantic Ridge). Measured temperatures of the disturbed borehole reached 91.3 degrees C, and equilibrated temperatures of the deepest section are estimated to be between 110 - 140 degrees C. Water collected every similar to 5-m during drilling operations had H-2 concentrations that were regularly > 200 nM and spiked to > 10 mu M at multiple depths. In these waters, carbon monoxide was only present in deeper, hotter sections, and potentially associated with gabbroic intrusions into the peridotite host. Open borehole fluids were sampled after drilling and samples recovered from the deepest portion contained elevated short-lived 222-Radon and strontium isotope ratios similar to Lost City fluids, pointing to the presence of in situ subseafloor formation waters that have equilibrated with the host rock. The deepest samples were actively degassing upon recovery and contained 740 +/- 360 mu M H-2, 340 +/- 36 mu M CH4, and 25.5 mu M & sum;formate (= formate and formic acid). The shallowest fluids from the open borehole also contain micromolar H-2 and & sum;formate concentrations, the presence of which cannot be attributed to the upward migration of the deeper, higher concentration fluids. We interpret these data as reflecting two distinct and interconnected regimes of fluid flow and composition. Deep waters that are channelized along faults, lithologic contacts, and other high permeability pathways host high H-2 and CH4 concentrations plus micromolar & sum;formate that closely mirror the chemistry and isotopic signatures of LCHF vent fluids. Pervasive fluid flow permeates the mesh texture and microfracture network of the serpentinized peridotite and sustains H-2 and & sum;formate even in the shallowest subseafloor intervals at mild temperatures. These findings demonstrate that both focused and pervasive fluid flow contribute to the transport, and potentially the generation, of reduced volatiles and C1 compounds within the Atlantis Massif.

The upper mantle is critical for our understanding of terrestrial magmatism, crust formation, and element cycling between Earth’s solid interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution have been primarily inferred by surface sampling and indirect methods. We recovered a long (1268-meter) section of serpentinized abyssal mantle peridotite interleaved with thin gabbroic intrusions. We find depleted compositions with notable variations in mantle mineralogy controlled by melt flow. Dunite zones have predominantly intermediate dips, in contrast to the originally steep mantle fabrics, indicative of oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the full depth of the core and is overprinted by oxidation in the upper 200 meters. Alteration patterns are consistent with vent fluid composition in the nearby Lost City hydrothermal field.

New high-resolution bathymetric data from Atlantis Massif and surrounding seafloor (30°N, Mid-Atlantic Ridge) records avolcanic extension associated with the formation of the axial rift valley floor, following the tectonic truncation of an active corrugated oceanic detachment fault system. The truncated Atlantis detachment is tectonically uplifted by a high-angle valley-bounding normal fault, formed after westward migration of the ridge at ∼0.4 to 0.1 Ma. Detachment fault remnants, with preserved corrugations, lie within the present-day rift valley seafloor, and demonstrate that a ∼20 km ridge section in the immediate vicinity of the Atlantis Fracture zone has not recorded any recent volcanic activity. Avolcanic extension may thus occur locally at the slow-spreading Mid-Atlantic Ridge, albeit for limited periods of time (less than a few hundred thousand years). The new fault dissecting the detachment shows a throw of ∼2800 m, partly due to flexural uplift. Emplacement of the Lost City hydrothermal site occurred at a late stage post-dating the detachment truncation and avolcanic rift valley formation. From the inferred timing of the westward ridge axis shift we calculate uplift rates ≥ 7 mm/yr, possibly as high as 33 mm/yr, which are equivalent to or greater than the fastest vertical uplift rates of active normal faults measured to date on Earth (Gulf of Corinth). Geomorphologic observations also demonstrate that mass wasting efficiently reworks the seafloor topography. We obtain local incision and erosion rates ≥1-2 mm/yr locally, and as high as 4-8 mm/yr, depending on the assumed age for the rift bonding fault (0.4 vs. 0.1 Ma respectively). Our results suggest that (1) avolcanic extension may occur locally at the slow-spreading Mid-Atlantic Ridge, albeit for limited periods of time (less than a few 100s of kyrs), and (2) document that shifts in axial valley location related to the abrupt abandonment of detachment faults is a first-order process in the asymmetric accretion of slow-spread oceanic lithosphere.

When hydrothermal activity ceases at black-smoker chimneys on mid-ocean ridges, populations of associated invertebrates hosting chemoautotrophic endosymbionts decline and then disappear, but the chimneys can persist on the seabed as relicts. Suspension-feeding brisingid seastars colonize hydrothermally inactive (relict) chimneys on the East Pacific Rise (EPR), though their distribution relative to available hard substrata and proximity to hydrothermal activity is poorly documented. In this study, brisingid abundance on sulfide and basalt substrata was assessed along an ∼3,700 m ROV Jason II transect at the summit of Pito Seamount (SE Pacific; ∼2,275 m). Brisingids were non-randomly distributed, with highest densities (up to ∼300 m –2 ) on relict sulfides chimneys near active black smokers. Brisingids were relatively uncommon on basalt substrata, and absent on black smokers. We infer that both relict sulfide structures and proximity to black smokers play key roles in the maintenance of dense brisingid populations on Pito Seamount and in similar environments on the EPR. Our observations suggest that experimental introduction of “artificial” relict chimneys providing microtopographic relief could test whether such an approach might mitigate potential impacts of mineral extraction on populations of suspension-feeding invertebrates.

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed similar to 10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30 degrees N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (<0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, 'cryptic' melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59-61 wt%) and low MgO/SiO2 values (0.48-0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of similar to 20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Earth’s magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years 1 . In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations 2 – 4 , geospeedometry 5 – 7 and thermal modelling 8 – 10 of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike–gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike–gabbro transition that extend 7–8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis 2 , 7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust.

G. Kimura, T. Hirose, M. Ikari, K. Kanagawa, M. Kinoshita, H. Kitajima, D. Saffer, H. Tobin, A. Yamaguchi, N. Eguchi, L. Maeda, S. Toczko, J. Bedford, S. Chiyonobu, T.A. Colson, M. Conin, P.H. Cornard, A. Dielforder, M.-L. Doan, J. Dutilleul, D.R. Faulkner, R. Fukuchi, G. Guérin, Y. Hamada, M. Hamahashi, W.-L. Hong, A. Ijiri, D. Jaeger, T. Jeppson, Z. Jin, B.E. John, M. Kitamura, A. Kopf, H. Masuda, A. Matsuoka, G.F. Moore, M. Otsubo, C. Regalla, A. Sakaguchi, J. Sample, A. Schleicher, H. Sone, K. Stanislowski, M. Strasser, T. Toki, T. Tsuji, K. Ujiie, M.B. Underwood, S. Yabe, Y. Yamamoto, J. Zhang, Y. Sanada, Y. Kido, E. Le Ber, and S. Saito with contributions by T. Kanamatsu2

Abstract Magnetic surveys at tectonic windows that expose magnetic polarity boundaries provide the unique opportunity to explore the pattern of magnetization variations within the oceanic crust and determine the spatially averaged magnetizations of source layers that contribute to marine magnetic anomalies. Here we investigate the C2An.2n/C2An.2r polarity boundary in the tectonic window of Pito Deep, which has exposed a cross‐section through lavas, dikes, and the uppermost kilometer of gabbros at fast‐spread ocean crust. Near‐bottom magnetic anomaly surveys from two expeditions have been incorporated into a penalized least squares inversion method. The application of this method to magnetic data allows us to account for complex bathymetry and differing observation altitudes. When correlated with rock type, the magnetization solution shows median values of 4.4 ± 2.7 A/m for lavas, 2.0 ± 1.9 A/m for dikes, and 1.9 ± 1.9 A/m for gabbros. On a regional scale, lavas and dikes have a different polarity of magnetization than the underlying gabbros. The geometry of the polarity boundary is compatible with a large (~6 km) horizontal offset or very shallow dip of isotherms at the dike/gabbro boundary, and indicates that the zone of melt is significantly wider across axis than predicted from seismic tomography models that suggest pervasive cooling throughout the lower crust within a few kilometers of the spreading center.

The tectonic window at Pito Deep, in the southern Pacific Ocean, permits study of the formative processes of uppermost East Pacific Rise (EPR) gabbroic ocean crust. Here we present a detailed microstructural and crystallographic study of 17 gabbroic samples fromthe uppermost similar to 800m of plutonic crust exposed in the Pito Deep Rift. We integrate two-and three-dimensional measurements of crystal size, shape, spatial distribution and orientation, with petrographic observations and geochemical data to constrain the formation of fast spread gabbroic ocean crust. The shallowest samples, collected < 55 metres below the sheeted dikes (mbsd), have evolved bulk-rock compositions, elongate plagioclase crystals, a clear plagioclase shape- and crystallographic-preferred orientation, and preserve only minor amounts of intracrystalline strain. The characteristics of these rocks and their proximity to the sheeted dike complex, suggests they formed by crystallization at the lateral tip of an axial melt lens that solidified as it moved off axis. Underlying samples from 96-724 mbsd, record more primitive bulk-rock compositions, less elongate plagioclase crystals and exhibit increasing strength of both plagioclase shape- and crystallographic-preferred orientation with depth below the sheeted dikes. These samples host plagioclase crystals that show increasing intracrystalline strain with depth, suggesting magmatic to hypersolidus submagmatic flow within the mush zone beneath the axial melt lens. These observations, together with inclined-to-steeply dipping mineral layering preserved below similar to 180 mbsd, are interpreted to record the downward transport of crystal-rich magma originating at the bottom of the melt lens through the uppermost kilometre of the mush zone at the EPR. The location of initial crystallization along the floor of the axial melt lens determines the magmatic processes that affect the crystal-rich magma en route to solidification as lower ocean crust.